FR2656552A1 - PROCESS FOR THE PRODUCTION OF THIXOTROPIC METALLIC PRODUCTS BY CONTINUOUS CASTING WITH ELECTROMAGNETIC CURRENT POLYPHASE CURING. - Google Patents

Abstract

The invention relates to a continuous casting method for producing thixotropic metallic alloys containing degenerated dendrites. It consists of casting the liquid metal in a movable occluded mould consisting of a hot upstream zone produced from insulating material and a cooled downstream zone in which the metal solidifies, while carrying out by means of a sliding magnetic field, obtained by a series of polyphase inductors, an electromagnetic agitation which causes the dendrites formed in the cold zone to pass into the hot zone where they change to nodules by superficial refusion. This invention is particularly applicable to the production of aluminum plates or bars having thixotropic properties.

Description

i X

  PROCESS FOR PRODUCING THIXOTROPIC METAL PRODUCTS

  BY CONTINUOUS CASTING WITH ELECTROMAGNETIC BREWING

CURRENT POLYPHASE

DESCRIPTION OF THE PRIOR ART

  The invention relates to a process for producing thixotropic metal products by continuous casting of plates or billets of

  circular, elliptical or polygonal section.

  A thixotropic metal product is a metallic product having a non-dendritic primary phase consisting in particular of dendrites

  degenerated into roughly spherical nodules.

  These thixotropic products offer advantages over conventional products when they are shaped: the shaping energy is lower, the cooling time is shorter, the shrinkage is reduced, the wear of the molds and dies is attenuated. Several patents teach means for obtaining thixotropic metal products: US Patent 3948650 (equivalent to French patent 2141979) describes a casting process consisting of raising the temperature of a composition to the liquid state, to cool to cause partial solidification and stir vigorously the liquid-solid mixture to break the dendrites and turn them into substantially spherical degenerate nodules representing up to about 65% by weight of the composition

initial.

  US Pat. No. 4,434,837 describes an electromagnetic stirring device applied to the continuous casting of thixotropic metal in which a two-pole stator creates a magnetic field rotating in a plane perpendicular to the axis of the mold and directed towards this axis L The interaction of this field with the induced current in the metal parallel to this axis generates electromagnetic forces located in a horizontal plane and tangential to the mold causing a shear rate of at least 500 sec. US Pat. No. 4,457,355 discloses a formed mold. two parts of different thermal conductivity and the European patent EP 71822 an ingot mold formed of a succession of insulating and conductive sheets. US Patent 4482012 describes an improvement consisting in using an ingot mold formed of two chambers interconnected by a non-conductive seal, the first of which acts as a heat exchanger and in US Pat. No. 4,565,241, it has been recommended that the conditions be met. agitation such that the ratio of the shear rate to the solidification rate is understood

3 3

between 2 10 and 8 10

SUMMARY OF THE INVENTION

  The prior art mentioned above and in particular the conditions fixed at the shear rate show that the essential problem encountered by the inventors is that the electromagnetic forces applied to the metal being solidified are sufficient to break up the already formed dendrites and to transform them. in nodules The degeneration of dendrites into nodules is obtained by mechanical effect only, and not by thermal effect Indeed, the forces applied to dendrites are, in the case of round billets, tangential to concentric circles centered on the axis of the gold mold, these circles are, because of the symmetry of revolution of the mold and the cast product,

isotherms.

  The present invention consists in obtaining the transformation of the dendrites into nodules by causing a remelting of the surface of these dendrites by a continuous transfer of the cold zone where they are formed towards a warmer zone. 'an effect

essentially thermal.

  In the process of the invention, the liquid metal is poured into a mold of vertical or horizontal axis with a moving bottom consisting of: a so-called hot upstream part whose wall or at least its internal face is made of an insulating material the heat of a so-called cold downstream portion whose wall made, at least partially, of a heat conducting material is in contact

  outside with a cooling fluid.

  In the upstream part, thermally insulated, the poured metal remains liquid; in the cooled downstream part, as in a known manner in continuous casting, a solidification front shown in FIG. 1 is formed by the reference 7 immediately upstream of this front, there is a liquid-solid zone consisting of primary crystals of suspension in

metal still liquid.

  The invention consists in printing on this liquid metal during solidification a movement represented by the arrow of FIG. 1, ensuring a transfer of the primary crystals from the cold zone to the hot z 8 in a time less than or equal to 1 second to cause a surface reflow of the dendritic crystals and transform them into

degenerate nodules.

  This transfer movement is provided by one or more inductors

  polyphase whose arrangement is explained below.

  The continuous casting apparatus for implementing the invention is shown in vertical half-section in FIG. 1. This is an example illustrating a particular embodiment of the invention, that of a casting. vertical; similar devices having the same functions are found in another embodiment of the invention

that of a horizontal casting.

  The device for the casting, the solidification and the extraction of the metal will be successively described and the device

  electromagnetic device to ensure the circulation of the metal.

  The casting, solidification and extraction device is close to that used for the continuous casting (or with raising) of metals and in particular of aluminum It comprises: a) a hot part made of thermal insulating material 1 which contains the liquid metal 2 The insulating material is of the type commonly used

  foundry for the manufacture of chutes and nozzles.

  b) a cold part connected to the hot part in a liquid-metal-tight manner This cold part comprises as essential element an externally cooled heat-conducting metal mold 3 This cooling can be provided by a film of cooling fluid, generally water, 5 from a water box 6, as shown in Figure 1 It can also be provided directly by a water chamber contiguous in known manner to the ingot mold In the latter case, it will preferably form jets or a blade of water at the bottom of the water-ingot mold chamber which will ensure by run-off the direct cooling of the product during casting This mold can be provided at its upper part with a graphite ring 4 a role of lubricant vis-à-vis the cast metal, in addition to a lubricating agent which it is sometimes necessary to coat the wall

  internal part of the downstream part to facilitate the casting of certain metals.

  According to a technique of the prior art, it is also possible to continuously supply a lubrication agent through the ingot mold which passes through the graphite ring and thus ensures continuous lubrication. Always with a view to continuous lubrication, it is possible to instead of using a graphite ring, to insert into the ingot mold graphite rods which open on the inner face of the mold and are connected at their other end to a chamber where the

lubricant is pressurized.

  c) the extraction system consists of a bottom closing the lower part of the ingot mold at the start, carried in the case of a vertical casting, by a plateau animated with a regular vertical movement and towards

  the bottom, adjustable according to the alloys and the format of the cast products.

  In the case of horizontal casting, this system is carried either by a motorized belt, or by a roller table, one of which is motorized.

  pressure roller to guarantee the drive.

  After starting, the already solidified product serves as a mold for the solidification of the fed metal continuously and we arrive at a state

  equilibrium shown in FIG.

  From the mold is developed a solidified outer casing while inside the cast product is established a solidification front 7 having the approximate shape shown in the figure. Below this front, the metal is completely solid; above, in what is called the "marsh", there is a mixture of liquid and solid particles, generally dendritic, particles that gradually integrate with the solidification front allowing the part

  solid 8 to grow and to progress.

  The electromagnetic stirring device whose combination with the casting device constitutes the invention consists of one or more inductors fed polyphase current surrounding the entire casting device, z 8 not hot and cold zone In principle, it is possible to use any type of current N phases, in practice we will obviously use the three-phase current: this is what is illustrated on the

figure 1.

  In this figure, six successive windings: A, B, C, D, E, F have been represented from top to bottom of the figure. These coils are placed in planes perpendicular to the casting axis They are powered in a manner analogous to that of the supply of the linear induction motors respectively by the phases 1, -2, 3, -1, 2, -3 so as to create a sliding vertical field ascending, descending or periodically ascending and descending, according to the order in which are fed the three phases In a conventional manner, a winding length which is a multiple of the polar pitch is generally used, the polar pitch being the length of the three windings of the elementary sequence 1, -2, 3 This means in practice 6, 9, 12, 15 coils will be used, but nothing in theory prevents the cutting of a sequence in the middle. Moreover, the means are known to those skilled in the art of electrical engineering consisting of using particular coils at the ends. in order to eliminate the edge effects in linear motors The interaction of this sliding field with the currents induced in the metal generates forces that induce movements in planes passing through the axis of the mold and thus of the cast product These movements are schematically represented on the

Figure 1 by the arrow 10.

  It is clear that these movements allow the entrainment of the dendrites formed near the solidification front and embedded in the still liquid metal towards the warmer upper part of the marsh where they undergo a superficial melting transforming them into nodules, then again towards the cold zone in the direction of the arrow 11. The three-phase inductors A to F described above can be made in two ways: 1010) conventionally with coils in the form of uncooled copper wire slabs or copper tube cooled, the various wafers being superimposed and preferably placed in the notches of a laminated magnetic yoke 12 for channeling the lines of force of the magnetic field (Figure 2) The metal sheets, electrically insulated from each other, are located in planes passing through the axis of the mold In the case where a water chamber is contiguous to the mold to ensure cooling of it, the windings can be placed inside this chamber.

thus effectively cooled.

  ) according to the invention as shown in Figure 3: The winding consists of thin discs of the order of mm of good conductor metal, copper for example Each disc 13 in the form of a ring and provided with a slot 14 constitutes a turn of the winding (FIG. 3 a). To constitute the winding, the discs are stacked by shifting two successive discs by a given angle. A sheet of insulator 15 is interposed between two successive copper discs except in the zone between the slots of these two successive discs There is therefore in this zone a contact area ensuring the connection

  between two successive disks and the continuity of the winding.

(Figure 3b).

  To ensure the cooling of the copper discs, we have imagined

  two solutions according to the cooling mode of the mold.

  In the case of cooling by water film of the mold, the windings according to the invention described above are placed in an annular water chamber surrounding the upstream and downstream parts and leaving the passage for the water film at mold level A

  half-section of this chamber is shown in Figure 4.

  The chamber consists of an inner wall, cylindrical or prismatic, depending on the shape of the cast product, in terms of preferably insulating or weakly electrically conductive, but in any case nonmagnetic 16 and an outer wall, with internal surface also cylindrical or prismatic, which may consist of a cylinder head

magnetic 12.

  The water chamber is closed at its upper and lower parts by two parts 18 and 19 joined together by a threaded tie rod 20 whose role is also to tighten the discs forming the winding with two nuts 21 and 22 Sa central part, in contact

  with the copper discs is isolated.

  A water inlet 23 and an outlet 24 are respectively disposed at the bottom and at the top of the chamber. The stacks of copper disks 13 and of insulator 15 are arranged inside the chamber. Copper and insulator disks are pierced with holes, judiciously distributed to form the cooling ducts 25 and allow the passage

tie rods.

  In the case where the cooling of the mold is provided by a water chamber, one can take advantage of this solution by placing the

  windings directly into this chamber (Figure 5).

  The inner wall of the chamber is in this case the mold itself 26 which is coated upstream (hot part) of a thermally insulating material 27 and downstream (cold part) of a graphite ring 28. solid water 29 of the product being cast delimited by the front 30 The water that arrives at 31 serves not only to cool the coils 32 via the holes 33, and the mold by passing through the gap 34, but also to forming a water slide 35 which flows on the product A valve 37 located on the outlet circuit 38 of the winding cooling enables the respective quantities circulating in each of the passages to be controlled. The rest of the technology is similar to what has been

described for Figure 4.

EXAMPLE.

  A billet of diameter 150 mm in aluminum alloy of the type A 57 G 03 (Al-7% Si-0.3% Mg), modified with strontium, was cast according to the method

described above.

  The casting speed was 150 mm / minute. The casting temperature was 6401 C. The upstream part of the mold consisted of a ring of refractory material 145 mm in internal diameter and 100 mm in height. The downstream part had the shape classic of an ingot mold for the casting of this type of alloy in this diameter A cylindrical linear motor of total winding height 180 mm surrounded these two parts It consisted of 9 coils, each made using 15 copper discs thickness 1 mm, internal diameter 200 mm and outer diameter 300 mm The assembly was supplied with three-phase star mounting and the applied voltage between

phases was 15 volts.

  A micrographic examination at the heart of the billet showed that a degenerated dendritic structure with a

  typical of globules close to 60 gm.

Claims (14)

  1) Process for the continuous casting of thixotropic metal products and, in particular, of aluminum alloy products, at least part of which of which is in the form of nodules originating from degenerate dendrites, into which the metal is poured liquid in a mold closed at its downstream part by a movable bottom and consisting of two parts of the same axis: a) an upstream portion, said z 8 not hot (1) whose wall is made of thermal insulating material at least on its face internal b) a downstream portion, said cold zone (3) whose wall, made of thermal conductive material is cooled by a cooling fluid (5) so as to cause in the liquid metal the appearance of primary crystals, the formation of contact of the cooled wall of a solid crust and a solidification front (7) allowing the gradual extraction of the product using the movable bottom characterized in that it is arranged around the heating zones. and cold mold a series of inductors consisting of annular windings having axis of the mold axis and supplied with polyphase current with a phase order as is established inside the mold a sliding field parallel to the axis of the mold moving in one direction or the other or alternately in one direction and then in the other, generating electromagnetic forces driving the metal in movements in planes passing through the axis of the mold and ensuring the transfer of the crystals dendritic from the cold zone to z 8 hot (10) and vice versa (11) to cause a superficial reflow of these crystals and their degeneration into nodules.  302) Method according to claim 1 characterized in that the casting continuous is done vertically.   3) Process according to claim 1 characterized in that the casting continuous is done horizontally.   4) Device for the continuous casting of thixotropic metal products comprising a mold closed at its downstream part by a movable bottom and consisting of two parts of the same axis: an upstream portion, said z 8 not hot (1) whose wall is made of thermal insulating material at least on its inner face a downstream portion, said cold zone (3) whose wall, made of thermal conductive material is cooled by a cooling fluid (5), a moving bottom moving system for extraction progressive product characterized in that are arranged around z 8 nes hot and cold mold a series of inductors consisting of annular windings having axis axis of the mold and supplied polyphase current with a phase order such that s' establish inside the mold a sliding field parallel to the axis of the mold moving alternately in one direction then in the other, generating electromagnetic forces located s in planes passing through the axis of the mold.   ) Device according to claim 4 characterized in that the axis of the mold is vertical.   6) Device according to claim 4 characterized in that the axis of the mold is horizontal.   257) Device according to one of claims 4 to 6 characterized in that   the inner wall of the ingot mold located in the cold zone 8 is provided over its entire periphery and at its most upstream portion of a ring   graphite of the same axis as that of the mold.   3 8) Device according to one of claims 4 to 6 characterized in that   the inner wall of the mold located in the cold zone is traversed over its entire periphery and at its most upstream part by graphite rods which bring the lubricant to the inner wall of the mold.   9) Device according to one of claims 4 to 8 characterized in that he   the windings are fed with three-phase current.   O) Device according to one of claims 4 to 9 characterized in that   the total length of the windings is a multiple of the polar pitch.   11) Device according to any one of claims 4 to 10 characterized   in that the cooling system of the downstream part is constituted   a film from a water box.   12) Device according to any one of claims 4 to 10   characterized in that the cooling system of the downstream part is constituted by a water chamber contiguous to the mold and forming at its base jets or a plate of water streaming on the cast product.   13) Device according to any one of claims 4 to 12 characterized   in that the coils are made in the form of slabs   superimposed copper wire or cooled copper tube.   2014) Device according to claim 13 characterized in that the patties   are placed in the notches of a laminated magnetic yoke.   ) Device according to one of claims 4 to 12 characterized in that   the windings consist of thin annular metal disks, preferably copper, each of them being provided with a radial slot, these disks being stacked with each successive disk a shift of a given angle of the slot and with interposition between two neighboring disks of an insulating sheet except in the zone between the slots of two adjacent disks so as to create a contact area between these   two discs and thus ensure the continuity of the winding.   16) Device according to claim 15 characterized in that the windings thus formed are placed inside a chamber traversed by a water circulation, of annular shape surrounding the casting mold, provided with clamping means of the stack of copper disc and insulating sheets and in that these copper discs and insulation are pierced with aligned holes allowing the circulation of cooling water.   17) Device according to claim 16 characterized in that the wall   external chamber consists of a magnetic yoke.   18) Device according to one of claims 12 to 14 characterized in that   that, to ensure their cooling, the windings are placed at   the interior of the water chamber contiguous to the ingot mold.   19) Device according to claims 12 and 16 characterized in that the   chamber for cooling the stacks of copper disks and insulation is constituted by the water chamber for cooling the cast product, whose inner wall is the   ingot mold itself coated upstream of a thermal insulating material.